Ion implantation has been a critical technology in semiconductor device manufacturing and is currently used for many processes including fabrication of the p-n junctions in transistors, particularly for CMOS devices such as memory and logic chips. By creating positively-charged ions containing the dopant elements required for fabricating the transistors in silicon substrates, the ion implanters can selectively control both the energy (hence implantation depth) and ion current (hence dose) introduced into the transistor structures. Traditionally, ion implanters have used ion sources that generate a ribbon beam of up to about 50 mm in length. The beam is transported to the substrate and the required dose and dose uniformity are accomplished by electromagnetic scanning of the ribbon across the substrate, mechanical scanning of the substrate across the beam, or both. In some cases, an initial ribbon beam can be expanded to an elongated ribbon beam by dispersing it along a longitudinal axis. In some cases, a beam can even assume an elliptical or round profile.
Currently, there is an interest in the industry in extending the design of conventional ion implanters to produce a ribbon beam of larger extent. This industry interest in extended ribbon beam implantation is generated by the recent industry-wide move to larger substrates, such as 450 mm-diameter silicon wafers. During implantation, a substrate can be scanned across an extended ribbon beam while the beam remains stationary. An extended ribbon beam enables higher dose rates because the resulting higher ion current can be transported through the implanter beam line due to reduced space charge blowup of the extended ribbon beam. To achieve uniformity in the dose implanted across the substrate, the ion density in the ribbon beam needs to be fairly uniform relative to a longitudinal axis extending along its long dimension. However, such uniformity is difficult to achieve in practice.
In some beam implanters, corrector optics has been incorporated into the beam line to alter the ion density profile of the ion beam during beam transport. For example, Bernas-type ion sources have been used to produce an ion beam of between 50 mm to 100 mm long, which is then expanded to the desired ribbon dimension and collimated by ion optics to produce a beam longer than the substrate to be implanted. Using corrector optics is generally not sufficient to create good beam uniformity if the beam is greatly non-uniform upon extraction from the ion source or if aberrations are induced by space-charge loading and/or beam transport optics.
In some beam implanter designs, a large-volume ion source is used that includes multiple cathodes aligned along the longitudinal axis of the arc slit, such that emission from each cathode can be adjusted to modify the ion density profile within the ion source. Multiple gas introduction lines are distributed along the long axis of the source to promote better uniformity of the ion density profile. These features attempt to produce a uniform profile during beam extraction while limiting the use of beam profile-correcting optics. Notwithstanding these efforts, the problem of establishing a uniform ion density profile in the extracted ion beam remains one of great concern to manufacturers of ribbon beam ion implanters, especially when utilizing ion sources having extraction apertures dimensioned in excess of 100 mm. Therefore, there is a need for an improved ion source design capable of producing a relatively uniform extracted ion beam profile.
Another shortcoming of traditional ion implanters is that they have ion sources made mostly of refractory metals. However, such metallic ions sources can produce ion beams containing contaminants (e.g., refractory metal compounds) that are difficult to remove even with the aid of sophisticated mass selection approaches. Hence, at least some contaminants are transported and implanted onto a workpiece. If the workpiece is a silicon wafer used in the fabrication of integrated circuits, the presence of even a few parts-per-million (ppm) of the contaminants can negatively affect yield.
Since ion beams are composed of positively-charged ions, positive charge can build up on an implanted workpiece, potentially damaging the devices which populate the workpiece. To implement charging control, an electron flood is typically deployed near the workpiece. Such electron floods are devices which emit copious amounts of low-energy electrons. The low-energy electrons can propagate directly to the workpiece, and can also be trapped by the positive potential of the ion beam and carried to the workpiece by the ion beam. Modern implanters typically use “plasma electron floods,” which are similar in construction to ion sources. However, unlike an ion source, a plasma electron flood's purpose is to produce low-energy electrons in sufficient quantities to compensate for positive charging of the workpiece during implantation. A common type of plasma electron flood incorporates a thermionic filament, which historically is composed of a refractory metal such as tungsten. In a typical plasma electron flood, due to its proximity to the substrate, tungsten evaporated from the hot filament can contaminate the substrate during flood operation. If the workpiece is a silicon wafer used in the fabrication of integrated circuits, the presence of even a few parts-per-million (ppm) of refractory metal contaminants can negatively affect yield.
In recent years, plasma electron floods have been introduced that do not contain a thermionic filament, but rather use microwave excitation or radio-frequency (RF) excitation to produce the plasma. While such floods may not produce refractory metal contamination, they are expensive, relatively large, and complex to design and operate.